Various types of glow plugs have been proposed; one such type is described in U.S. Pat. No. 2,130,365. The present invention relates to this type of glow plug. This type of glow plug includes an electrical heating winding which is constructed as an elongated spiral made of molybdenum or tungsten wire embedded into an insulating, heat-conductive powder or granulate, typically magnesium oxide or aluminum oxide. If such a spiral heating wire is made of tungsten or molybdenum, and the embedding insulating material is made of magnesium oxide, it has been found that the heating wire spiral may tear since the thermal coefficient of expansion of magnesium oxide is substantially greater than that of tungsten or molybdenum, respectively. If, however, such heating wire spirals are embedded in aluminum oxide, then the preheating time needed by such preheating plugs or glow plugs is substantially increased due to the relatively poor heat conductivity of aluminum oxide. The increase in preheating time is unacceptable for modern automotive-type internal combustion engines.